The present invention relates to a back-off tool adapted to be lowered into a pipe string in a well bore to apply an explosive shock at a threaded coupling between pipes in the pipe string so as to jar the coupling so that torque applied to the pipe string at the surface will "back off" the threads at that coupling to uncouple the string at that location.
The usual prior art device employed to accomplish this result comprises lengths of detonating cord taped to a central steel rod which is lowered by wireline into the pipe string, then electrically detonated at the desired location. This prior art method, known as a "string shot", leaves tape debris in the well and requires side detonation from cord to cord, which not only is somewhat unreliable, but produces a ragged, non-uniform explosion which may or may not produce a shock wave of the necessary magnitude and uniformity. Moreover, the detonating cord used in the prior art device must be shipped at a high cost due to explosive shipping regulations, because of the amount of explosive in a length of the cord.
U.S. Pat. No. 2,911,909 discloses a droppable back-off tool which is actuated by the impact of the tool on a sub previously placed at a particular location downhole. This has the disadvantage of not being able to effect the explosion where desired, as well as restricting the bore of the pipe. In addition, there is no assurance that the tool impact will be great enough to actuate the detonator. Moreover, the tool employs detonating cord which must be somehow maintained in a groove on the tool exterior, the means for such maintenance not being disclosed.
U.S. Pat. No. 4,007,790 discloses a back-off apparatus which relies upon a non-destructive explosive to provide a jarring force to a pipe coupling through mechanical jarring means. The nature or configuration of the explosive is not disclosed. Moreover, the explosive force which may be applied to the pipe coupling is limited by the reusable nature of the apparatus.
Another prior art device which appears superficially to be similar to the back-off tool of the present invention is disclosed in U.S. Pat. No. 3,174,545. The patent discloses a device employed in a method of explosive-induced hydraulic fracturing, which device includes an explosive contained in a frangible housing, which explosive is detonated by electrically actuated detonating means. However, the device as disclosed produces an irregular, pulsating explosion or series of explosions to generate fluid pressure pulsations, and fragments of the destroyed housing are designed to plug some of the perforations in the well bore casing so that the hydraulic force will act more strongly against others. This device is obviously unsuitable for use as a back-off tool due to the design of the explosive charge as well as the relatively large nature of the housing fragments produced as a result of the explosive detonation, which fragments could inhibit subequent drilling, well treatment and production.